NULL vs nullptr (Why was it replaced?)

nullptr has type std::nullptr_t. It's implicitly convertible to any pointer type. Thus, it'll match std::nullptr_t or pointer types in overload resolution, but not other types such as int.

0 (aka. C's NULL bridged over into C++) could cause ambiguity in overloaded function resolution, among other things:

f(int);
f(foo *);

(Thanks to Caleth pointing this out in the comments.)

Here is Bjarne Stroustrup's wordings,

In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days.

If you have to name the null pointer, call it nullptr; that's what it's called in C++11. Then, "nullptr" will be a keyword.

You can find a good explanation of why it was replaced by reading A name for the null pointer: nullptr, to quote the paper:

This problem falls into the following categories:

• Improve support for library building, by providing a way for users to write less ambiguous code, so that over time library writers will not need to worry about overloading on integral and pointer types.

• Improve support for generic programming, by making it easier to express both integer 0 and nullptr unambiguously.

• Make C++ easier to teach and learn.